'Play' icon for run-cell toolbar button

Based on @ellisonbg's research in gh-2914, most tools with similar
functionality to run a piece of code use either the 'play' icon, or
something custom designed (RStudio and Spyder are in the latter camp).

The analogy with audio and video does suggest that 'step forward' is
more similar to running one cell, but it's not clear that users
are actually thinking about it in those terms.

I'm opening this for discussion rather than for immediate merge.

Author: takluyver

docs/source/examples/Notebook/Notebook Basics.ipynb

@@ -192,9 +192,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "The second idea of mouse based navigation is that **cell actions usually apply to the currently selected cell**. Thus if you want to run the code in a cell, you would select it and click the <button class='btn btn-default btn-xs'><i class=\"fa fa-step-forward icon-step-forward\"></i></button> button in the toolbar or the \"Cell:Run\" menu item. Similarly, to copy a cell you would select it and click the <button class='btn btn-default btn-xs'><i class=\"fa fa-copy icon-copy\"></i></button> button in the toolbar or the \"Edit:Copy\" menu item. With this simple pattern, you should be able to do most everything you need with the mouse.\n",
+    "The second idea of mouse based navigation is that **cell actions usually apply to the currently selected cell**. Thus if you want to run the code in a cell, you would select it and click the <button class='btn btn-default btn-xs'><i class=\"fa fa-play icon-step-forward\"></i></button> button in the toolbar or the \"Cell:Run\" menu item. Similarly, to copy a cell you would select it and click the <button class='btn btn-default btn-xs'><i class=\"fa fa-copy icon-copy\"></i></button> button in the toolbar or the \"Edit:Copy\" menu item. With this simple pattern, you should be able to do most everything you need with the mouse.\n",
     "\n",
-    "Markdown cells have one other state that can be modified with the mouse. These cells can either be rendered or unrendered. When they are rendered, you will see a nice formatted representation of the cell's contents. When they are unrendered, you will see the raw text source of the cell. To render the selected cell with the mouse, click the <button class='btn btn-default btn-xs'><i class=\"fa fa-step-forward icon-step-forward\"></i></button> button in the toolbar or the \"Cell:Run\" menu item. To unrender the selected cell, double click on the cell."
+    "Markdown cells have one other state that can be modified with the mouse. These cells can either be rendered or unrendered. When they are rendered, you will see a nice formatted representation of the cell's contents. When they are unrendered, you will see the raw text source of the cell. To render the selected cell with the mouse, click the <button class='btn btn-default btn-xs'><i class=\"fa fa-play icon-step-forward\"></i></button> button in the toolbar or the \"Cell:Run\" menu item. To unrender the selected cell, double click on the cell."
    ]
   },
   {

docs/source/examples/Notebook/Running Code.ipynb

@@ -25,7 +25,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Run a code cell using `Shift-Enter` or pressing the <button class='btn btn-default btn-xs'><i class=\"icon-step-forward fa fa-step-forward\"></i></button> button in the toolbar above:"
+    "Run a code cell using `Shift-Enter` or pressing the <button class='btn btn-default btn-xs'><i class=\"icon-step-forward fa fa-play\"></i></button> button in the toolbar above:"
    ]
   },
   {

notebook/static/notebook/js/actions.js

@@ -148,7 +148,7 @@ define([
         },
         'run-cell-and-select-next': {
             cmd: i18n.msg._('run cell and select next'),
-            icon: 'fa-step-forward',
+            icon: 'fa-play',
             help: i18n.msg._('run cell, select below'),
             help_index : 'ba',
             handler : function (env) {